Since the inception of atomic force microscopy (AFM), it has become the standard technique in imaging various sample surfaces down to the nanometer scale in ambient or fluid medium. Compared with scanning tunneling microscopes (STM), AFM is commonly employed for imaging nano sized objects or structures since it is applicable to all materials while STM can only be used for conducting or semi-conducting materials. Besides its capability to characterize surfaces in nanometer scale, it has been demonstrated recently that AFM can be employed to modify surfaces and manipulate nano sized structures. For instance, by using AFM, thin oxide structures have been rearranged on the underlying surface by increasing applied load while scanning. The sled-type motion of C60 islands during imaging has been studied by Y Kim and C. M. Lieber as reported in “Machining Oxide Thin Films with an Atomic Force Microscope: Pattern and Objective Formation on the Nanometer Scale”, Science Vol. 257:375-377, 1992. In “Sled-type Motion on the Nanometer Scale: Determination of Dissipation and Cohesive Energies of C60”by R. Luthi, E. Meyer, H. Haefke, L. Howald, W. Gutmannsbauer and H. J. Guntherodt, Science, Vol. 266:19779-1981, 1994, it was demonstrated that AFM can be used to deliberately move an Au cluster on a smooth surface. The applications of AFM to manipulate and position nanometer-sized particles with nanometer precision was discussed by T. Junno, K. Deppert, L. Montelius and L. Samuelson in “Controlled Manipulation of Nanoparticles with an Atomic Force Microscope”, Applied Physics Letters, Vol. 55:3627-3629, June 1995. Using AFM to construct arbitrary patterns of gold nanoparticles was reported by A. A. G. Requicha, C. Baur, A. Bugacov, B. C. Gazen, B. Koel, A. Madhukar, T. R. Ramachandran, R. Resch, and P. Will in “Nanorobotic Assembly of Two-dimensional Structures” Proc. IEEE Int. Conf. Robotics and Automation, 3368-3374, May 1998. In these experiments, the samples were first imaged in non-contact mode to minimize the lateral force acting on the samples and then manipulation was carried out with the normal force feedback switched off. A problem with this process is that the normal force cannot be controlled during manipulation, as the force feedback is switched off. This might result in either breaking cantilevers due to large normal force or insufficient force to keep the tip in contact with the surface. Another problem of this manipulation scheme is that it can only manipulate two-dimensional nanostructures on a smooth substrate surface. The surface tilt must be carefully removed before manipulation. One possible solution for this problem is to use techniques developed for three-dimensional nanomanipulation.
A promising method for 3-D nanomanipulation is to build a small nanomanipulator inside the vacuum capsule of a scanning electron microscope (SEM). Piezoelectric manipulators constructed inside the SEM have the ability to manipulate objects along the three linear degrees of freedom using the AFM tip as the end-effectors. Several kinds of manipulation of carbon nanotubes were performed using this device. The obvious advantage of this method is that multi-end effectors can be built inside the SEM to achieve more degrees of freedom. The manipulation can be performed between the end-effectors without the need of a substrate. The operation can also be monitored in real time. However, the manipulation accuracy of this method is not comparable to using the AFM. Since the samples are placed in vacuum and exposed to electron beam with high energy, this manipulator cannot be used to manipulate biological samples. The expense of a SEM, ultrahigh vacuum condition, and space limitation inside the SEM vacuum capsule also impede the wide application of this method.
Another scheme was proposed by L. T. Hansen, A. Kuhle, A. H. Sorensen, J. Bohr and P. E. Lindelof in “A Technique for Positioning Nanoparticles using an Atomic Force Microscope”, Nanotechnology, Vol. 9:337-342, 1998. In this scheme, the nanoparticles were imaged in tapping mode and pushed over a surface in contact mode, while the normal feedback was switched on. This method can be considered as the beginning of the 3-D nanomanipulation in the sense that the cantilever tip follows the topography of the surface using the internal feedback control. However, this method takes the risk of hazardous action of the feedback mechanism when switching on and off the vibration of the cantilever, changing the gains of the feedback loop, changing other parameters such as setpoint, tip velocity, while the tip is touching the sample surface.
Recently, some researchers are trying to combine AFM with virtual reality interface and haptic devices to facilitate the nanomanipulation. By introducing a virtual environment of the samples, nanomanipulation using the AFM becomes much easier. Besides three-dimension synthetic visual feedback to users, a one degree of freedom haptic device has been constructed for haptic sensing by M. Sitti and H. Kashimoto in “Tele-nanorobotics using Atomic Force Microscope”, Proc. IEEE Int. Conference Robotics and Automation 1739-46 October 1998. In the manipulation mode, during approaching to and contacting the surface of the object, or manipulating the object, the operator controls the x-y-z motion of the cantilever while feeling the normal tip-sample interaction force. However, because only the normal force is “feelable” during manipulations, this method may take the risk of breaking cantilever and damaging the tip when large lateral forces occur. These methods can also be considered as the 3-D nanomanipulation since the cantilever tip can be controlled either by the internal force feedback control loop or by the operator through the haptic device to follow the topography of the surface.
Therefore, it is desirable to an improved method for performing nanomanipulation operations using an atomic force microscope. By carefully modeling the sample surface within the AFM frame, the arbitrary and feasible motion paths of the tip are obtainable through three-dimensional path planning. Furthermore, it is desirable to detected the forces acting upon the tip during a nanomanipulation operation. The forces may then be used to provide real-time feedback to an operator of the atomic force microscope. This allows the operator adjust the paths in real-time to ensure sufficient force without damaging the tip of the objects under manipulation.